The Assassin and the Pirate Lord (Throne of Glass #0.1)

Descriptions:

A Throne of Glass novella.

On a remote island in a tropical sea, Celaena Sardothien, feared assassin, has come for retribution. She’s been sent by the Assassin’s Guild to collect on a debt they are owed by the Lord of the Pirates. But when Celaena learns that the agreed payment is not in money, but in slaves, her mission suddenly changes—and she will risk everything to right the wrong she’s been sent to bring about.